Normal cells have robust mechanisms to ensure high fidelity genome replication and segregation. By contrast, the majority of cancer cells exhibit genomic instability resulting from mutations within these control systems. Recent studies strongly indicate, however, that cells lacking the p53 cell cycle control pathway must also be exposed to conditions that promote DNA damage to induce large-scale chromosomal alterations. This raises the important question of whether mutations arising during tumor progression might generate biochemical changes that destabilize the genome. We propose to use sensitive microscopic, molecular, biochemical and genetic methods to investigate the hypothesis that activation of oncogenes during tumorigenesis produces reactive oxygen species (ROS) that induce DNA damage. Systems to be analyzed involve activation of the c-Myc oncogene using both inducible systems, and cell lines with characterized defects in APC-catenin signaling. We will determine the mechanisms by which c-Myc induces ROS, as such studies could lead to development of more effective chemopreventive strategies for cancer. p53 is at the nexus of a critical tumor suppressor pathway in humans that limits emergence of maintaining genet variants, yet the precise mechanisms by which it functions are still hotly debated. Experiments are proposed to investigate p53 structure-function relationships in vivo using a mutant mouse we generated in which one of the p53 N-terminal transactivation domains was inactivated. This system will enable stringent tests of the hypothesis that there is more than one transactivation domain required for full p53 function in vivo. Preliminary results suggest that mutant mice exhibit a different tumor spectrum than p53 null mice. Studies to test this possibility rigorously are proposed. Additional experiments test the hypothesis that the stable, transcriptionally defective mutant protein functions as a dominant negative in vivo. The mutant p53 gene provides a framework for future studies to elucidate those subregions that participate in stress activated cell growth regulation and tumor suppression in multiple tissues in vivo. Taken together, these studies should add significantly to our understanding of how oncogenes drive tumor progression, and how p53 suppresses tumor function.